1. Field of the Invention
The field of art to which the claimed invention pertains is liquid-gas absorptive separations. More specifically, the claimed invention relates to a process for the absorptive removal of sulfur dioxide from a gaseous mixture by means of a scrubbing liquor.
2. Description of the Prior Art
Due to the ever increasing concern about air pollution, great efforts have been expended in recent years toward the development of processes to reduce the pollutants introduced into the atmosphere from various industrial operations. One of the most onerous of these pollutants is sulfur dioxide which is present in the stacks of flue gases from various operations. For example, SO.sub.2 is a primary pollutant released into the atmosphere in the production of sulfuric acid. Also, great quantities of SO.sub.2 are produced and passed to the atmosphere in the generating of power, particularly where the fuel used is high in sulfur content.
It has long been known to the art how to abate sulfur dioxide pollution of the atmosphere. The so-called "wet process" is probably the most commonly used process to accomplish such abatement. In the wet process the SO.sub.2 containing gas mixture is contacted or "scrubbed" with water in which is dissolved or slurried a reagent, particularly an alkaline reagent, that chemically reacts with the SO.sub.2, thus effecting the removal of the SO.sub.2 from the gas mixture and incorporating the sulfur originally contained in the SO.sub.2 into a non-volatile compound dissolved in the scrubbing water. The sulfur is ultimately removed from the system by the discarding of the scrubbing water in which it is contained, or by removal from the water by further processing.
The alkaline reagent perhaps best known to the art for use in the scrubbing solution for the above wet process is alkali metal carbonate or bicarbonate. The alkali metal carbonate or bicarbonate reacts with the SO.sub.2 to yield alkali metal sulfite or bisulfite and CO.sub.2. Use of the alkali metal carbonate and specifically sodium carbonate or bicarbonate is particularly advantageous because solutions of that compound are commonly available as an abundant mineral deposit (Trona) occurring in the western part of the United States, and as process-waste streams from industries such as metals production, textiles and paper making. The use of such streams not only cuts operating costs but also permits economies in the capital costs of wet processes because it eliminates the need for equipment to store and mix reagents obtained from other sources. Examples of prior art wet SO.sub.2 scrubbing processes which utilize alkali metal carbonates are taught or claimed in U.S. Pat. Nos. 3,962,410; 3,963,825; 3,972,980; 3,984,529; and 3,987,147.
It has been observed that certain inexpensive sources of sodium carbonate or bicarbonate (soda liquor), such as that produced as a byproduct from the production of commercial grade sodium carbonate, contain quantities of silica which, when the soda liquor is used in the wet scrubbing process, precipitate and form deposits on the internals of the scrubbing apparatus. These deposits, which are extremely hard solids, interfere with the operation of the apparatus by eventually plugging the pipes and liquid circulating means which are a part of the apparatus. There is no practical way of removing these deposits.
Effective means of minimizing the deleterious effects of the presence of silica in the alkali reagent used in the wet SO.sub.2 scrubbing process has been developed prior to this invention. Such means comprise the addition of a water soluble salt of a metal such as iron, cobalt, nickel, copper or aluminum to the soda liquor prior to the use of the soda liquor in the wet scrubbing process so as to effect a chemical reaction between the silica and salt to obtain a silicon containing material having a minimum deleterious effect when precipitated. I have discovered an improvement to these previously developed means which enables the minimization of the amount of salt required to be added to the soda liquor, or the maximization of the amount of silica that can be rendered innocuous by a given amount of salt.